Integrated optical mach zehnder structures

Abstract

An integrated optical Mach Zehnder structure has an optical path length modifying heater (phase-shifter) associated with a portion of the length of each interference arm of the Mach Zehnder. The portion of the length of the interference arm associated with one of the heaters is flanked with trenches that provide that portion with a temperature coefficient of birefringence different from that of the corresponding portion of the other interference arm. The invention is applicable no only to stand-alone Mach Zehnders, but also to more complex Mach Zehnder structures having a series combination of two or more component Mach Zehnders that are close coupled by having the two outputs of one component Mach Zehnder of the complex structure constitute the two inputs of the next component Mach Zehnder of the complex structure.

What is claimed is:  
     
       1. An integrated optical waveguide format optical device including at least one Mach Zehnder structure constituted by an optical input optically coupled with an optical output via two optical interference arms constituted respectively by first and second optical waveguides optically in parallel providing optical coupling between first and second optical waveguide splitter/combiners, wherein a first length portion of the first optical waveguide, and a second length portion of the second optical waveguide, are provided respectively with first and second optical path length modifying heaters, and wherein the first length portion is furnished with a structure that provides the first length portion with a temperature coefficient of birefringence different from that of the second length portion. 
     
     
       2. An integrated optical waveguide format optical device as claimed in claim  1 , wherein the optical input and the optical output are optically coupled with the two interference arms by respective first and second four-port directional couplers. 
     
     
       3. An integrated optical waveguide format optical device as claimed in claim  1 , wherein the optical input and the optical output are optically coupled with the two interference arms by respective first and second four-port directional couplers. 
     
     
       4. An integrated optical waveguide format triple Mach Zehnder comprising a central Mach Zehnder close coupled between a matched pair of Mach Zehnders comprising an input end Mach Zehnder and an output end Mach Zehnder, each of said central and end Mach Zehnders having first and second waveguides that are optically coupled in an input coupling region and an output coupling region and having first and second interference arms constituted respectively by the regions of the first and second waveguides extending between the input and output coupling regions, wherein, to provide the close coupling between the central and end Mach Zehnders, the input coupling region of the central Mach Zehnder also constitutes the output coupling region of the input end Mach Zehnder, and the output coupling region of the central Mach Zehnder also constitutes the input coupling region of the output end Mach Zehnder, wherein, in respect of the central Mach Zehnder, a first length portion of the first interference arm and a second length portion of the second interference arm are provided respectively with first and second optical path length modifying heaters, and wherein the first length portion is furnished with a structure that provides the first length portion with a temperature coefficient of birefringence different from that of the second length portion. 
     
     
       5. An integrated optical waveguide format triple Mach Zehnder as claimed in claim  4 , wherein the input end, and the output end, Mach Zehnders each have first and second interference arms of fixed optical path length difference. 
     
     
       6. A method of attenuating in a dynamically adjustable manner an optical signal having a spectral spread about a central wavelength, in which method the signal is transmitted through a triple Mach Zehnder as claimed in claim  5  wherein, in respect of the interference arms of the input end and output end Mach Zehnders, the fixed optical path difference is substantially equal to a value corresponding to a phase difference of π/2 at the central wavelength. 
     
     
       7. A method as claimed in claim  6 , wherein in the absence of the application of any drive to the first and second optical path length modifying heaters, the interference arms of the central Mach Zehnder differ in optical path length by an amount that compensates at least partially wavelength sensitivity of the triple Mach Zehnder that results from wavelength dependent coupling strength effects in its optical waveguide splitter/combiners. 
     
     
       8. A method as claimed in claim  7 , wherein the first and second optical path length modifying heaters are differentially driven in a manner providing the interference arms of the central Mach Zehnder with a differential birefringence that compensates at least partially polarisation state sensitivity of the triple Mach Zehnder that results from polarisation state dependent coupling strength effects in its optical waveguide splitter/combiners. 
     
     
       9. An integrated optical waveguide format triple Mach Zehnder as claimed in claim  4  wherein, in respect of each end Mach Zehnder, a first length portion of the first interference arm, and a second length portion of the second interference arm, are provided respectively with first and second optical path length modifying heaters, and wherein the first length portion is furnished with a structure that provides the first length portion with a temperature coefficient of birefringence different from that of the second length portion. 
     
     
       10. An integrated optical waveguide format triple Mach Zehnder as claimed in claim  9  wherein, in respect of each end Mach Zehnder, the first and second interference arms differ in optical path length by an amount corresponding to a phase difference that is adjustable, by means of the optical path length modifying heaters of that end Mach Zehnder, within a range lying between π/2 and π. 
     
     
       11. A method of operating an integrated optical waveguide format triple Mach Zehnder as claimed in claim  10  to provide it with an optical filter characteristic having an absorption characteristic of desired centre wavelength and optical density, wherein first and second drives are applied to the optical path length modifying heaters respectively of the central and end Mach Zehnders respectively for controlling the centre frequency and the optical density, and wherein polarisation dependent effects in the end Mach Zehnders are compensated at least in part by adding, to the application of the first drive to the optical path length modifying heaters of the central Mach Zehnder, a third drive that is a linear function of the second drive.